For what type of people can money truly buy happiness?

The emotionally unstable.

Via The Calgary Herald:

Reporting in the journal Social Psychological and Personality Science, researchers find compelling evidence that money predicts life satisfaction far more strongly for the highly neurotic — those prone to mood swings, anxiety or negativity — than for people who are more emotionally stable.

And:

“At high levels of income, pretty much everyone is happy; at low levels of income, stable people are still perfectly satisfied with their lives, but the neurotic people are very dissatisfied,” says Soto, an assistant professor of psychology at Colby College in Maine. “Money is a much more powerful predictor of happiness for highly neurotic individuals.”